Genome-wide meta-analysis of infant developmental milestones and temperament
Temperament and Milestones
Temperament broadly refers to individual differences in behaviour that are typically measurable in infancy and early childhood. Broad dimensions include domains such as, negative affect and surgency. Measures typically capture a large number of individual subscales, as well as more general overall domains.
Fortunately, measurement of infant temperament has been developed over several decades, with a considerable number of psychometric studies to support the measures and with an emphasis on capturing reliable individual differences. Commonly used scales include the infant temperament scales by Carey  and the Infant–Toddler Social and Emotional Assessment by Carter and Briggs-Gowan, and those developed originally by Mary Rothbart, Samuel Putnam and colleagues (including the Infant Behaviour Questionnaire, the Early Childhood Behaviour Questionnaire) e.g. . Measures of Milestones include the Denver Development Scales and the Infant Monitoring Scale .
Temperament reflects early development of personality and behaviour. Infant temperament predicts a variety of later outcomes including childhood psychiatric disorders , bedwetting in middle childhood  liability for ASD [6, 7] and ADHD symptoms . Developmental milestones, such as motor milestones like age at first step, are considered important signposts that help to indicate infant development.
Twin heritability for temperament has tended to be reported as between 30-40% (e.g. [9, 10], with some studies not finding significant heritability e.g. on positive affect . In general, this research field is characterised by smaller twin studies compared to studies of older ages. Some of the very large developmental twin cohorts of >5000 pairs (e.g. TEDS, CATSS) have either tended to begin main assessments after infancy or have included a small number of items in infancy.
Molecular Genetic studies
In reviews by our group, we reported no GWAS on infant temperament to date [12, 13]. Most of the past literature used the candidate gene design.
Justification and Scientific Gap
The behaviour of infants impacts parents and thus their ability to parent - the relationship between child and parent is dynamic and bidirectional. Infant behavior also impacts marital relationships and other siblings. Infant behavior and development significantly predicts later outcomes in childhood such as neurodevelopment, personality and mental health [1-5]. Infant behaviour impacts many aspects of society, either directly or indirectly. Underlying infant behavior are key changes in brain development but we know very little about what causes variation in infant behavior and development.